Surface Scanning Inspection System With Independently Adjustable Scan Pitch

ABSTRACT

A surface scanning wafer inspection system with independently adjustable scan pitch and associated methods of operation are presented. The scan pitch may be adjusted independently from an illumination area on the surface of a wafer. In some embodiments, scan pitch is adjusted while the illumination area remains constant. For example, defect sensitivity is adjusted by adjusting the rate of translation of a wafer relative to the rate of rotation of the wafer without additional optical adjustments. In some examples, the scan pitch is adjusted to achieve a desired defect sensitivity over an entire wafer. In other examples, the scan pitch is adjusted during wafer inspection to optimize defect sensitivity and throughput. In other examples, the scan pitch is adjusted to maximize defect sensitivity within the damage limit of a wafer under inspection.

CROSS REFERENCE TO RELATED APPLICATION

The present application for patent is a continuation of, and claimspriority under 35 U.S.C. §120 from U.S. patent application Ser. No.13/413,521, entitled “Surface Scanning Inspection System With AdjustableScan Pitch,” filed Mar. 6, 2012, which, in turn, claims priority under35 U.S.C. §119 from U.S. provisional patent application Ser. No.61/451,592, entitled “Method of Using Pitch Adjustment in SSIS forSensitivity and T-put Optimization,” filed Mar. 10, 2011, the subjectmatter of each are incorporated herein by reference in their entireties.

TECHNICAL FIELD

The described embodiments relate to systems for wafer inspection, andmore particularly to scanning modalities in wafer inspection.

BACKGROUND INFORMATION

Semiconductor devices such as logic and memory devices are typicallyfabricated by a sequence of processing steps applied to a substrate orwafer. The various features and multiple structural levels of thesemiconductor devices are formed by these processing steps. For example,lithography among others is one semiconductor fabrication process thatinvolves generating a pattern on a semiconductor wafer. Additionalexamples of semiconductor fabrication processes include, but are notlimited to, chemical-mechanical polishing, etch, deposition, and ionimplantation. Multiple semiconductor devices may be fabricated on asingle semiconductor wafer and then separated into individualsemiconductor devices.

Inspection processes are used at various steps during a semiconductormanufacturing process to detect defects on wafers to promote higheryield. As design rules and process windows continue to shrink in size,inspection systems are required to capture a wider range of physicaldefects on wafer surfaces while maintaining high throughput.

One such inspection system is a spinning wafer inspection system thattranslates and rotates a wafer in a fixed ratio during inspection. Thefixed ratio is selected to provide the desired defect sensitivity overthe entire wafer under inspection. Improvements to spinning waferinspection systems are desired to both maintain desired defectsensitivity and improve throughput.

SUMMARY

A scanning wafer inspection system with independently adjustable scanpitch and associated methods of operation are presented. The scan pitchmay be adjusted independently from an illumination area on the surfaceof a wafer. In some examples, scan pitch is adjusted while theillumination area remains constant. In some embodiments, defectsensitivity is adjusted by adjusting the rate of translation of a waferrelative to the rate of rotation of the wafer without additional opticaladjustments.

In some examples, the scan pitch is adjusted to achieve a desired defectsensitivity over an entire wafer. By adjusting the scan pitch, defectsensitivity may be controlled by parameter adjustments within a motioncontroller rather than complex optical actuation.

In another example, the scan pitch is adjusted during wafer inspectionto optimize defect sensitivity and throughput. By adjusting the scanpitch during a wafer scan, the defect sensitivity may be matched to thenoise signature of the wafer surface over all locations on the waferresulting in increased throughput.

In yet another example, the scan pitch is adjusted to maximize defectsensitivity within the damage limit of a wafer under inspection. Byadjusting scan pitch defect sensitivity may be increased while utilizinga relatively large beam size. The large beam size lowers the powerdensity on the wafer surface resulting in a reduced risk of thermal orphoto-chemical damage.

The foregoing is a summary and thus contains, by necessity,simplifications, generalizations and omissions of detail; consequently,those skilled in the art will appreciate that the summary isillustrative only and is not limiting in any way. Other aspects,inventive features, and advantages of the devices and/or processesdescribed herein, as defined solely by the claims, will become apparentin the non-limiting detailed description set forth herein.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a plot illustrative of a spinning wafer inspection system 100.

FIG. 2 is a simplified diagram illustrative of a wafer 102 underinspection and an illumination area 102 a.

FIG. 3 is a plot 310 illustrative of the defect sensitivity as afunction of track pitch for a Surfscan® SP3 wafer inspection systemmanufactured by KLA-Tencor Corporation, San Jose, Calif.

FIG. 4 is a plot 320 illustrative of a simulation of the normalizedfalse count onset threshold as a function of changed in scan pitch.

FIG. 5 illustrates a plot 330 that highlights simulation resultsindicating the beam intensity profiles of different beam sizes at waferlevel.

FIG. 6 illustrates a plot that highlights simulation results indicatingchanges in beam intensity at the track edges based on scan pitchadjustment for a fixed beam size.

FIG. 7 illustrates a plot 340 that highlights simulation resultsindicating sensitivity and scan pitch as a function of scan radius.

FIG. 8 illustrates a plot 350 of simulation results indicatingthroughput improvement that may result from increasing scan pitch as afunction of wafer radius in the manner illustrated in FIG. 7 for anumber of exemplary scenarios.

FIG. 9 is a flowchart illustrative of a method 400 of adjusting a scanpitch independently from an illumination area of a scanning waferinspection system.

DETAILED DESCRIPTION

Reference will now be made in detail to background examples and someembodiments of the invention, examples of which are illustrated in theaccompanying drawings.

FIG. 1 is a simplified schematic view of a typical wafer inspectionsystem 100. For simplification, some optical components of the systemhave been omitted, such as components directing the illumination beamsto the wafer. A wafer 102 is illuminated by any of a normal incidencebeam 104 and an oblique incidence beam 106 generated by one or moreillumination sources 101. The area or spot 102 a illuminated by eitherone or both beams 104, 106 on wafer 102 scatters radiation from thebeam(s). The radiation scattered by area 102 a along directions close toa line 116 perpendicular to the surface of the wafer and passing throughthe area 102 a is collected and focused by lens collector 118 anddirected to a photo-multiplier tube (PMT) 120. Since lens 118 collectsthe scattered radiation along directions close to the normal direction,such collection channel is referred to herein as the narrow channel andPMT 120 as the dark field narrow PMT. When desired, one or morepolarizers 122 may be placed in the path of the collected radiation inthe narrow channel.

Radiation scattered by spot 102 a of wafer 102, illuminated by eitherone or both beams 104, 106, along directions away from the normaldirection 116 is collected by an ellipsoidal collector 124 and focusedthrough an aperture 126 and optional polarizers 128 to dark field PMT130. Since the ellipsoidal collector 124 collects scattered radiationalong directions at wider angles from the normal direction 116 than lens118, such collection channel is referred to as the wide channel. Theoutputs of detectors 120, 130 are supplied to a computer 132 forprocessing the signals and determining the presence of anomalies andtheir characteristics.

Various aspects of surface inspection system 100 are described in U.S.Pat. No. 6,271,916 and U.S. Pat. No. 6,201,601, both of which areincorporated herein by reference. An exemplary surface inspection systemis available from KLA-Tencor Corporation of San Jose, Calif., theassignee of the present application.

In one embodiment, wafer positioning system 125 includes a wafer chuck108, motion controller 114, a rotation stage 110 and a translation stage112. Wafer 102 is supported on wafer chuck 108. As illustrated in FIG.2, wafer 102 is located with its geometric center 150 approximatelyaligned the axis of rotation of rotation stage 110. In this manner,rotation stage 110 spins wafer 102 about its geometric center at aspecified angular velocity, ω, within an acceptable tolerance. Inaddition, translation stage 112 translates the wafer 102 in a directionapproximately perpendicular to the axis of rotation of rotation stage110 at a specified velocity, V_(T). Motion controller 114 coordinatesthe spinning of wafer 102 by rotation stage 110 and the translation ofwafer 102 by translation stage 112 to achieve the desired scanningmotion of wafer 102 within wafer inspection system 100.

Any of beams 104 and 106 illuminate an illumination area 102 a of wafer102 that is located a distance, R, from the geometric center of wafer102. Illumination area 102 a is defined (i.e., shaped and sized) by theprojection of any of beams 104 and 106 onto the surface of wafer 102.Illumination area 102 a may be interchangeably termed the beam spot sizeor the spot size of wafer inspection system 100.

In an exemplary operational scenario, inspection begins withillumination area 102 a located at the geometric center 150 of wafer 102and then wafer 102 is rotated and translated until illumination area 102a reaches the outer perimeter of wafer 102 (i.e., when R equals theradius of wafer 102). Due to the coordinated motion of rotation stage110 and translation stage 112, the locus of points illuminated byillumination area 102 a traces a spiral path on the surface of wafer102. The spiral path on the surface of wafer 102 is referred to as aninspection track 103 (not shown). Portions 103 a, 103 b, and 103 c of anexemplary inspection track 103 are illustrated in FIG. 2 as TRACK_(i+1),TRACK_(i), and TRACK_(i−1), respectively. The distance between adjacentportions of an inspection track (e.g., distance between TRACK_(i+1) andTRACK_(i)) is referred to as the scan pitch of the wafer inspectionsystem 100.

Typically, wafer inspection systems employ a fixed relationship betweenscan pitch and illumination area during operation. Thus, in typicalwafer inspection systems, scan pitch is not adjusted independently fromillumination area. Rather, in typical wafer inspection systems, whenscan pitch is adjusted, illumination area is scaled according to a fixedrelationship. For example, typical wafer inspection systems may offerseveral operational modes (e.g., fast, low resolution scan and slow,high resolution scan). Each mode may utilize a different scan pitch, butthe illumination area (i.e., spot size) corresponding with each mode isalso adjusted such that a fixed relationship between spot size and scanpitch is maintained for each mode. The Surfscan® SP1 wafer inspectionsystem manufactured by KLA-Tencor Corporation, San Jose, Calif., is anexample of a wafer inspection system that does not adjust scan pitchindependently from illumination area.

In one aspect, the scan pitch of wafer inspection system 100 is adjustedindependently from illumination area. In one example, the scan pitch isadjusted to achieve a desired defect sensitivity over an entire wafer102. In another example, the scan pitch is adjusted during inspection ofwafer 102. In one operational example, the scan pitch of a wafer underinspection is continuously adjusted as a function of the distancebetween the illumination area 102 a and the geometric center 150 ofwafer 102. In yet another example, the scan pitch is adjusted tomaximize defect sensitivity within the damage limit of wafer 102.

Referring to FIG. 1, wafer inspection system 100 includes a processor141 and an amount of computer readable memory 142. As depicted in FIG.1, by way of example, motion controller 114 includes processor 141 andmemory 142, however, processor 141 and memory 142 may be included inother components of wafer inspection system 100. Processor 141 andmemory 142 may communicate over bus 143. Memory 142 includes an amountof memory 144 that stores a program code that, when executed byprocessor 141, causes processor 141 to coordinate the motion of rotationstage 110 and translation stage 112 such that the ratio between the rateof translation of translation stage 112 and the rate of rotation ofrotation stage 110 (i.e., scan pitch) is adjusted. In one example, theratio between the rate of translation of translation stage 112 and therate of rotation of rotation stage 110 is adjusted as a function of thescan radius, R, between illumination area 102 a and the geometric center150 of wafer 102.

In addition, wafer inspection system 100 may include peripheral devicesuseful to accept inputs from an operator (e.g., keyboard, mouse,touchscreen, etc.) and display outputs to the operator (e.g., displaymonitor). Input commands from an operator may be used by processor 141to generate coordinated motion profiles of rotation stage 110 andtranslation stage 112. The resulting coordinated motion profiles may begraphically presented to an operator on a display monitor.

Achieving a desired defect sensitivity is a primary performanceobjective of a wafer inspection system. Defect sensitivity may bemeasured in terms of false count onset of the wafer inspection system.In one example, false count onset occurs when a wafer inspection systemreports a defect at a particular location on the wafer where no defectin fact exists (e.g., the defect is a noise artifact).

In general, to avoid false count onset and increase defect sensitivity,a wafer inspection system should operate with a high Signal to NoiseRatio (SNR). For a given laser power P, wafer rotational speed, w, andradial scan distance, r, measured from the scan spot to the geometriccenter of the wafer, the Signal to Noise Ratio (SNR) over anillumination area 102 a can be estimated as illustrated in Equation 1,where R_(T) is the tangential spot size and R_(R) is the radial spotsize.

$\begin{matrix}{{SNR} \propto {\sqrt{\frac{LaserPower}{\omega \; {RR}_{T}}}\left( \frac{1}{R_{R}} \right)}} & (1)\end{matrix}$

The relationship illustrated in Equation 1 relates the sensitivity ofSNR over the illumination area 102 a to spot size (i.e., R_(T) andR_(R)), laser power, and scan speed (i.e., ωR). Exemplary techniques formanaging illumination energy based on adjusting spot size, beamintensity, and spin rate are described in U.S. patent application Ser.No. 11/127,280 by applicant KLA-Tencor Corp. published on Nov. 16, 2006in U.S. Patent Publication No. US2006/0256325 A1, the entirety of whichis incorporated herein by reference.

However for situations where the detection sensitivity is limited bywafer surface noise and other detection noise, the inventors havediscovered that defect sensitivity is strongly coupled to scan pitch.The inventors have found that under these circumstances false countonset is particularly driven by SNR at the edge of the inspection track.Due to the energy distribution of a Gaussian beam on the wafer, changesin scan pitch have a significant effect on SNR at the track edge. Thus,defect sensitivity and throughput may be further optimized to meet userobjectives based on adjustment of scan pitch independently fromillumination area during wafer inspection.

The inventors have discovered that false count onset first occurs inportions of wafer 102 located near the track edge. Based on theappearance of these false counts, it appears as if a defect existshalfway between two adjacent scan tracks, when in fact, none exists.False count onset at the track edges (i.e., between adjacent tracklocations) is enhanced because the portions of wafer 102 located at thetrack edges is illuminated by the tails of a typical Gaussian radiationsource, rather than the peak of the Gaussian radiation source. In thisregion, defect detection is driven by combining defect signals of twoadjacent track locations. This combination compounds the detection noiseof the two adjacent track locations. As a result, for a given noisebackground, it is more likely that two moderate noise events arecombined to result in the onset of false counts.

FIG. 3 is a plot 310 illustrative of the defect sensitivity as afunction of track pitch for a Surfscan® SP3 wafer inspection systemmanufactured by KLA-Tencor Corporation, San Jose, Calif. Plot 310illustrates that false count onset is coupled to scan pitch at the edgeof a track. As illustrated in FIG. 3, the defect size associated withfalse count onset for both the wide detection channel and the narrowdetection channel are plotted as a function of track pitch. In all casesthe beam size was held constant. A range of scan pitch values rangingfrom 70% to 115% of the nominal scan pitch value were tested and theresults plotted in FIG. 3. As illustrated, a 10% change in defectsensitivity on the wide channel and a 6% change in defect sensitivity onthe narrow channel may result from adjusting the scan pitch within theillustrated range. Although FIG. 3 illustrates the sensitivity of falsecount onset on scan pitch in one non-limiting example, other operationalexamples illustrating different sensitivities may be contemplated.

FIG. 4 is a plot 320 illustrative of a simulation of the normalizedfalse count onset threshold as a function of changes in scan pitch. Twocases are illustrated. In one case, both the scan pitch and beam sizeare adjusted together in a fixed relationship. In the other case, scanpitch is adjusted independently from illumination area. Morespecifically, only scan pitch is adjusted and illumination area remainsconstant. As illustrated in FIG. 4, within a range between −10% and 20%of nominal scan pitch, false count onset performance based on changingscan pitch independently from illumination compared to changing bothscan pitch and beam size is practically the same. In other words, theexpense and complication of adjusting illumination area can be avoidedwhile controlling defect sensitivity by scan pitch adjustment alone.

Analogously, in certain situations, throughput can be improved byincreasing scan pitch without unacceptable losses in defect sensitivity.For inspection scenarios where the achieved inspection defectsensitivity is greater than a desired defect sensitivity, scan pitch canbe increased to improve throughput. In some embodiments, the scan pitchmay be increased without changing illumination area (as illustrated inFIG. 4) while maintaining a desired defect sensitivity. In this manner,throughput can be enhanced without a penalty on defect sensitivitysimply by scanning at a fixed illumination area with a larger scanpitch. Although FIG. 4 illustrates a scan pitch adjustment range between−10% and 20% of nominal scan pitch, other operational ranges may also becontemplated. For example, defect sensitivity may be tuned by adjustingscan pitch independently from illumination area within a range between−25% and 25% of nominal scan pitch. In another example, defectsensitivity may be tuned by adjusting scan pitch independently fromillumination area within a range between −50% and 50% of nominal scanpitch. In yet another example, defect sensitivity may be tuned byadjusting scan pitch independently from illumination area within a rangebetween −100% and 100% of nominal scan pitch.

FIG. 5 illustrates a plot 330 that highlights simulation resultsindicating the beam intensity profiles of different beam sizes at waferlevel. Each beam has the same total illumination power, only theillumination area is different. For example, beam intensity profile 151represents a nominal beam size, while beam intensity profiles 152 and153 represent beam sizes that are 30% smaller and 30% larger than thenominal beam size, respectively. Each beam intensity profile is plottedas a function of distance from the track center. A normalized trackposition equal to one represents the track edge. As illustrated,significant changes in beam size result in relatively small changes inbeam intensity near the track edge (e.g., at normalized track positionequal to one). Because, the beam intensity at the tails of a Gaussianshaped beam is relatively independent of beam size, it can be understoodthat changing beam size without a change in total illumination power hasa limited effect on false count onset at track edges. Moreover, precisecontrol of beam size does not yield significant improvements in falsecount onset at track edges. FIG. 6 illustrates the beam intensityprofile 151 illustrated in FIG. 5. As illustrated, small changes (e.g.,10%) in scan pitch result in significant changes in beam intensity nearthe resulting track edge. Moreover, precise control of scan pitch canyield significant improvements in false count onset at track edges.Thus, a wafer inspection system 100 that adjusts scan pitchindependently from illumination area is able to precisely control defectsensitivity vis-a-vis false count onset at track edges.

The cost of wafer inspection system 100 may be reduced because the costassociated with adjusting scan pitch as discussed herein is less thanthe cost associated with precise optical subsystems necessary for beamshaping. In particular, a wafer inspection system 100 may include alimited number of operational modes (e.g., three modes) and a beamshaping adjustment mechanism that only adjusts the beam shape whenchanging modes. Otherwise, when operating in a particular mode, all finetuning of defect sensitivity is achieved by adjustment of scan pitchwhile beam shape remains constant.

FIG. 9 is illustrative of a method 400 of adjusting a scan pitch of ascanning wafer inspection system in accordance with the embodimentspresented herein. At block 401, a wafer surface is illuminated over anillumination area. At block 402, the scan pitch of a scanning waferinspection system is adjusted independently from the illumination area.A number of operational scenarios may be contemplated based on ascanning wafer inspection system with scan pitch adjusted independentlyfrom illumination area.

In a first example, the scan pitch may be adjusted independently fromthe illumination area to meet a particular user requirement (e.g.,desired defect sensitivity). In this manner, the defect sensitivity ofthe wafer inspection system 100 may be adjusted for full wafer scans.This method would be used to adapt the system sensitivity to the userrequirements without optical adjustments. The scan pitch can be changedby operation of motion controller 114 as discussed herein. Moreover, inmany examples, no optical adjustment (e.g., zooming or power change) isrequired to achieve the desired defect sensitivity.

In a second example, the scan pitch may be adjusted during inspection ofa wafer to maintain sensitivity and boost throughput. As illustrated inFIG. 7, the fundamental sensitivity at the center of the wafer is higherthan at the edge. This occurs because the tangential scan velocity maybe limited by scan radius (i.e. distance between the illumination area102 a and the geometric center 150 of wafer 102) and the achievableangular velocity of rotation stage 110. Within this operating regime,sensitivity is increased due to reduced tangential scan velocity (seeEquation 1) and the opportunity to perform increased sample averaging.In a typical example, a limit on tangential scan velocity is reached tomaintain adequate sensitivity and this tangential scan velocity ismaintained to the perimeter of wafer 102. For example, this limit istypically reached at a scan radius that is approximately one half of thewafer radius.

In another aspect, the scan pitch of wafer inspection system 100 isincreased as the tangential scan velocity is decreased toward the centerof wafer 102. In the past, the sensitivity advantage near the center ofthe wafer was not exploited. However, an increase in scan pitch near thecenter portion of the wafer 102 that scales with the sensitivity profilewould tend to maintain a similar sensitivity throughout the wafer. Thisresults in an increase in throughput without loss of productionsensitivity. As illustrated in FIG. 7, the scan pitch is steadilyincreased by 20% from a scan radius where the tangential scan velocitybegins to be reduced (e.g., one half of the wafer radius) to thegeometric center of wafer 102. Thus, in general, the scan pitch of waferinspection system 100 is adjusted independently from illumination areabased at least in part on a distance between the geometric center of awafer and the illumination area to increase throughput while maintaininga desired defect sensitivity.

FIG. 8 illustrates simulation results indicating throughput improvementthat may result from increasing scan pitch as a function of wafer radiusin the manner illustrated in FIG. 7 for a number of exemplary scenarios.As illustrated, for a scan pitch adjustment of 20%, a 2-3% increase inthroughput may be realized. The results illustrated in FIG. 8 areprovided as a non-limiting illustrative example. Other operationalscenarios may be contemplated that result in different levels ofthroughput improvement.

Although, in the illustrated example, a limit on tangential scanvelocity is reached at a particular scan radius (e.g., half of the waferradius), this example is not meant to be limiting. Depending on systemparameters, a limit on tangential scan velocity may be reached at anyscan radius. In some examples, a limit on tangential scan velocity maynot be reached at all. In these examples, tangential scan velocity andscan pitch may be continuously scaled as a function of scan radius overthe entire wafer.

In a third example, scan pitch may be adjusted to maximize sensitivitywithin a damage limit of a wafer. As illustrated in Equation 1, as thebeam size and tangential scan velocity are decreased, the sensitivity ofwafer inspection system 100 increases. However, for a given amount ofincident illumination energy, additional gains in sensitivity cannot bemade by further decreases in either beam size or tangential scanvelocity. One reason is that the wafer surface would be damaged due toincident radiation energy exceeding the damage limit the wafer. Forexample, depending on materials and inspection conditions, an incidentpower density between one and ten milliwatts/μm² may bring a wafersurface near the damage limit. However, adjusting the scan pitch to asmaller value improves sensitivity without risk of wafer damage,effectively smearing the incident energy more evenly over the wafer. Inthis manner, defect sensitivity is further improved by decreasing thescan pitch.

In one or more exemplary embodiments, the functions described may beimplemented in hardware, software, firmware, or any combination thereof.If implemented in software, the functions may be stored on ortransmitted over as one or more instructions or code on acomputer-readable medium. Computer-readable media includes both computerstorage media and communication media including any medium thatfacilitates transfer of a computer program from one place to another. Astorage media may be any available media that can be accessed by ageneral purpose or special purpose computer. By way of example, and notlimitation, such computer-readable media can comprise RAM, ROM, EEPROM,CD-ROM or other optical disk storage, magnetic disk storage or othermagnetic storage devices, or any other medium that can be used to carryor store desired program code means in the form of instructions or datastructures and that can be accessed by a general-purpose orspecial-purpose computer, or a general-purpose or special-purposeprocessor. Also, any connection is properly termed a computer-readablemedium. For example, if the software is transmitted from a website,server, or other remote source using a coaxial cable, fiber optic cable,twisted pair, digital subscriber line (DSL), or wireless technologiessuch as infrared, radio, and microwave, the the coaxial cable, fiberoptic cable, twisted pair, DSL, or wireless technologies such asinfrared, radio, and microwave are included in the definition of medium.Disk and disc, as used herein, includes compact disc (CD), laser disc,optical disc, digital versatile disc (DVD), floppy disk and blu-ray discwhere disks usually reproduce data magnetically, while discs reproducedata optically with lasers. Combinations of the above should also beincluded within the scope of computer-readable media.

Although certain specific embodiments are described above forinstructional purposes, the teachings of this patent document havegeneral applicability and are not limited to the specific embodimentsdescribed above. In one example, wafer inspection system 100 may includemore than one light source (not shown). The light sources may beconfigured differently or the same. For example, the light sources maybe configured to generate light having different characteristics thatcan be directed to a wafer at the same or different illumination areasat the same or different angles of incidence at the same or differenttimes. The light sources may be configured according to any of theembodiments described herein. In addition one of the light sources maybe configured according to any of the embodiments described herein, andanother light source may be any other light source known in the art. Inanother example, wafer inspection system 100 may be a multi-spot system.In some embodiments, a multi-spot system may illuminate the wafer overmore than one illumination area simultaneously. The multipleillumination areas may spatially overlap. The multiple illuminationareas may be spatially distinct. In some embodiments, a multi-spotsystem may illuminate the wafer over more than one illumination area atdifferent times. The different illumination areas may temporally overlap(i.e., simultaneously illuminated over some period of time). Thedifferent illumination areas may be temporally distinct. In general, thenumber of illumination areas may be arbitrary, and each illuminationarea may be of equal or different size, orientation, and angle ofincidence. In yet another example, wafer inspection system 100 may be ascanning spot system with one or more illumination areas that scanindependently from any motion of wafer 102. In some embodiments anillumination area is made to scan in a repeated pattern along a scanline. The scan line may or may not align with the scan motion of wafer102. Although as presented herein, wafer positioning system 125generates motion of wafer 102 by coordinated rotational andtranslational movements, in yet another example, wafer positioningsystem 100 may generate motion of wafer 102 by coordinating twotranslational movements. For example motion wafer positioning system 125may generate motion along two orthogonal, linear axes (e.g., X-Ymotion). In such embodiments, scan pitch may be defined as a distancebetween adjacent translational scans along either motion axis. In suchembodiments, a wafer inspection system includes an illumination sourceand a wafer positioning system. The illumination source supplies anamount of radiation to a surface of a wafer over an illumination area.The wafer positioning system moves the wafer in a scanning motioncharacterized by a scan pitch (e.g., scanning back and forth in onedirection and stepping by an amount equal to the scan pitch in theorthogonal direction). The wafer positioning system includes a motioncontroller that adjusts the scan pitch independently from theillumination area. Accordingly, various modifications, adaptations, andcombinations of various features of the described embodiments can bepracticed without departing from the scope of the invention as set forthin the claims.

What is claimed is:
 1. A wafer inspection system comprising: anillumination source operable to supply an amount of radiation to asurface of a wafer over an illumination area; and a wafer positioningsystem for moving the wafer in a scanning motion characterized by a scanpitch, the wafer positioning system including, a motion controlleroperable to adjust the scan pitch independently from the illuminationarea to achieve a desired defect sensitivity over the wafer.
 2. Thewafer inspection system of claim 1, wherein the illumination arearemains constant as the scan pitch is independently adjusted.
 3. Thewafer inspection system of claim 1, wherein the scan pitch is adjustedwhile scanning the wafer.
 4. The wafer inspection system of claim 3,wherein the scan pitch is continuously adjusted from a first scan pitchto a second scan pitch over a portion of the wafer.
 5. The waferinspection system of claim 1, wherein the scanning motion involvessimultaneously rotating the wafer about a geometric center of the waferand translating the geometric center of the wafer, wherein a rate oftranslation of the wafer relative to a rate of rotation of the waferdetermines the scan pitch.
 6. The wafer inspection system of claim 5,wherein the scan pitch is adjusted based at least in part on a distancebetween the geometric center of the wafer and a location of theillumination area on the surface of the wafer.
 7. The wafer inspectionsystem of claim 1, wherein the scan pitch is adjusted based at least inpart on a desired defect sensitivity.
 8. The wafer inspection system ofclaim 7, wherein the scan pitch is adjusted to maintain the desireddefect sensitivity over the entire wafer with a maximum inspection speedfor the entire wafer.
 9. The wafer inspection system of claim 7, whereinthe scan pitch is adjusted to a maximum defect sensitivity within adamage limit of the wafer.
 10. A method of wafer inspection comprising:illuminating a surface of a wafer over an illumination area; andadjusting a scan pitch of a scanning wafer inspection systemindependently from the illumination area.
 11. The method of waferinspection of claim 10, wherein the adjusting of the scan pitch of thescanning wafer inspection system involves adjusting the scan pitch whilescanning the wafer.
 12. The method of wafer inspection of claim 10,wherein the scanning wafer inspection system is operable tosimultaneously rotate a wafer about a geometric center of the wafer andtranslate the geometric center of the wafer, and wherein the adjustingof the scan pitch involves changing a rate of translation of the waferrelative to a rate of rotation of the wafer.
 13. The method of waferinspection of claim 12, wherein the adjusting of the scan pitch is basedat least in part on a distance between the geometric center of the waferand a location of the illumination area on the surface of the wafer. 14.The method of wafer inspection of claim 10, wherein the adjusting of thescan pitch is based at least in part on maintaining a desired defectsensitivity.
 15. The method of wafer inspection of claim 14, wherein theadjusting of the scan pitch is based at least in part on maintaining thedesired defect sensitivity over the entire wafer with a maximuminspection speed for the entire wafer.
 16. The method of waferinspection of claim 14, wherein the adjusting of the scan pitch is basedat least in part on maximizing a defect sensitivity within a damagelimit of the wafer.
 17. A scanning wafer inspection system comprising:an illumination source operable to supply an amount of radiation to asurface of a wafer over an illumination area; and a non-transitory,computer-readable medium including, code for causing a computer toadjust a scan pitch of the scanning wafer inspection systemindependently from the illumination area.
 18. The scanning waferinspection system of claim 17, wherein the scan pitch of the scanningwafer inspection system is adjusted while scanning the wafer.
 19. Thescanning wafer inspection system of claim 17, wherein the scanning waferinspection system is operable to simultaneously rotate the wafer about ageometric center of the wafer and translate the geometric center of thewafer, and wherein the scan pitch is adjusted by changing a rate oftranslation of the wafer relative to a rate of rotation of the wafer.20. The scanning wafer inspection system of claim 19, wherein the scanpitch is adjusted based at least in part on a distance between thegeometric center of the wafer and a location of the illumination area onthe surface of the wafer.
 21. The scanning wafer inspection system ofclaim 17, wherein the scan pitch is adjusted based at least in part onmaintaining a desired defect sensitivity.
 22. The scanning waferinspection system of claim 17, wherein the scan pitch is adjusted basedat least in part on maintaining the desired defect sensitivity over theentire wafer with a maximum inspection speed for the entire wafer. 23.The scanning wafer inspection system of claim 17, wherein the scan pitchis adjusted based at least in part on maximizing a defect sensitivitywithin a damage limit of the wafer.